Group IVB boride based cutting tools for machining group IVB based materials

ABSTRACT

A densified titanium diboride based ceramic composition is provided having W and Co therein and a fine grain size. The composition has particular usefulness as a cutting tool for the machining of titanium based alloys at high speeds.

BACKGROUND OF THE INVENTION

The present invention relates to Group IVB (titanium, hafnium,zirconium) boride based articles, cutting tools and their densificationtechniques. It is especially related to titanium diboride based cuttingtools and their use to machine Group IVB metals and alloys, especiallytitanium and its alloys.

It was recognized as early as 1955 that "machining of titanium and itsalloys would always be a problem, no matter what techniques are employedto transform this metal into chips," (Siekmann H. J. Tool Engng, January1955, Vol. 34, Pages 78-82).

Over approximately the past forty years, commercial machining technologyfor most workpiece materials has advanced significantly. Ceramic, cermetand ceramic coated cutting tools have been developed and commercializedwhich have significantly improved productivity in machining of steels,cast irons and superalloys. However, during that same time period,progress in the field of machining titanium alloys has been minor. Thecommercial cutting tool materials of choice for most titanium machiningapplications remain high speed tool steels and an uncoated,approximately 6 weight percent cobalt cemented tungsten carbide, such asKennametal K313 cemented carbide grade. Where coated cemented carbidetools (e.g., Kennametal, KC720 and KC730 grades) have been applied totitanium alloy machining, they have met with only limited success. Theuse of uncoated cemented carbides to machine titanium based metallicmaterials has greatly limited productivity advances in the machining ofthese materials, since uncoated carbides are limited in most commercialapplications to speeds of 250 surface feet/minute or less when machiningtitanium alloys (see Dearnley et al., "Evaluation of Principal WearMechanisms of Cemented Carbides and Ceramics used for Machining TitaniumAlloy IMI318," Materials Science and Technology, January 1986, Vol. 2,Pages 47-58; Dearnley et al., "Wear Mechanisms of Cemented Carbides andCeramics used for Machining Titanium," High Tech Ceramics, ed. by P.Vincenzini, Elsevier Sci. Publ. (1987) Pages 2699-2712; Metals Handbook,Ninth Edition, Vol. 16, "Machining," (1989), Pages 844-857; Marchado etal., "Machining of Titanium and Its Alloys --A Review," Proc. Instn.Mech. Engrs., Vol. 204 (1990) Pages 53-60; and "Kennametal Tools,Tooling Systems and Services for the Global Metalworking Industry,"Catalogue No. A90-41(150)E1, (1991) Page 274.

Kennametal, KC, K313, KC720 and KC730 are trademarks of Kennametal Inc.,of Latrobe, Pennsylvania, for its cutting tool grades.

The machining speed used when machining titanium alloys with uncoatedcemented carbide tools may be increased to 500 to 1000 surfacefeet/minute, through the use of a high pressure coolant machining system(e.g., U.S. Pat. No. 4,621,547). These systems are expensive, difficultto integrate into existing machine tools, and require a significantamount of maintenance. Their application in titanium alloy machininghas, therefore, been limited.

Clearly, there has thus been an unfulfilled long-felt need for improvedcutting tool materials, and improved methods for machining titaniumbased metallic materials.

SUMMARY OF THE INVENTION

The present inventors have now surprisingly discovered a new cuttingtool material for machining titanium based metallic materials, whichsignificantly advances titanium machining productivity and fulfills thelong-felt need identified above. Applicants have found that the presentinvention may be utilized in the machining of a titanium alloy at ametal removal rate of about two to three times that obtained withuncoated carbide cutting tools using flood cooling while maintainingabout the same amount of metal removed per cutting edge. This results ina significant reduction in the labor time required to machine a giventitanium alloy workpiece while significantly increasing machineavailability. These results are achieved using standard flood coolingtechniques. The present invention, therefore, has the further advantagethat it does not require the use of a high pressure coolant system toachieve high machining speeds.

According to one aspect of the present invention, a method of chipforming machining (e.g., turning) of a titanium alloy is provided inwhich cutting is performed at a speed of at least 400, and morepreferably, at least 500, surface feet/minute with a cutting toolpreferably having a cutting edge lifetime of at least three minutes,while using flood cooling.

According to another aspect of the invention, a metalcutting tool forchip forming machining of Group IVB (Ti, Hf, Zr) metallic materials isprovided which has a rake face, over which chips of the Group IVBmetallic material will flow during machining, a flank face and a cuttingedge for cutting into said Group IVB materials at high speeds (≧400surface feet/minute). This metalcutting tool has a Group IVB boridebased (i.e., at least 60 w/o Group IVB borides) composition which,preferably, is a ceramic composition having a Group IVB boride phase,and preferably a second phase preferably formed as a residue of asintering aid and the Group IVB boride phase.

Preferably, the ceramic contains one or more phases of: N_(xn) M_(yn)boride, where x_(n) >Y_(n), Y_(n) ≧0, n is an integer≧1, N is titanium,hafnium or zirconium, alone or in solid solution with each other, and Mmay include W, Co, Mo, Ta, Nb, Fe, Ni, Al and/or Cr, but is preferably Wand/or Co. Preferably, the N_(x1) M_(y1) boride phase includes adiboride, and more preferably, a Ti_(x1) M_(y1) B₂ phase, and mostpreferably, TiB₂ crystal structure as determined by x-ray diffraction ofthe densified ceramic.

In a preferred embodiment in accordance with the present invention, theGroup IVB boride based densified ceramic composition has amicrostructure including a N_(x1) M_(y1) boride phase as describedabove, and having a second phase containing N and M (e.g., N_(x2) M_(y2)Z, where Z may be boron, or a borocarbide, boroxide, boronitride,borocarbonitride, oxyborocarbonitride boroxycarbide or boroxynitride).Preferably, a third phase also containing N and M (e.g., N_(x3) M_(y3)Z) is also present. Preferably, the ratio of y₂ /x₂ in the second phaseis greater than the ratio of y₃ /x₃ in the third phase, which is, inturn, preferably greater than y₁ /x₁ in the first phase. M may be any ofthe elements mentioned above with respect to the first boride phase, butpreferably includes tungsten and/or cobalt. Preferably, the second andthird phases form a matrix in which the N_(x1) M_(y1) boride phase isembedded. In many instances, the second phase is present as a haloaround the N_(x1) M_(y1) boride phase with the third phase outside ofthe second phase.

Minor phases that may be found in the microstructure of the presentinvention in addition to those already discussed include CoW₂ B₂, CoWB₅,WB, W₂ B, W₃ CoB, TiB and Ti₃ B₄.

It should be understood that the phases mentioned above may also containminor amounts of oxygen, carbon, nitrogen, and other elements fromsintering aids, toughening agents, grain refining agents and impurities.

The foregoing densified ceramic, in addition to its use as a cuttingtool in the high speed machining of reactive metals (i.e., Ti, Hf, Zr)and their alloys, may also be used to cut other materials (e.g.,aluminum and aluminum alloys, and hardened steels and hardened castirons), and may be used for non-cutting applications, as well. Suchnon-cutting applications include articles for handling, or which comeinto contact with, liquid metals, such as aluminum (e.g., boats,crucibles and electrodes) as well as plungers and dies for forming sheetmetal articles, such as cans.

According to another aspect of the present invention, a process isprovided for manufacturing the above titanium diboride based article ofmanufacture. This process includes the steps of adding to a TiB₂ powderan effective amount of Co and WC to substantially densify (i.e., atleast 97% of theoretical density) the material during sintering. It hasbeen found that, when Co and WC are added together to the presentcomposition in an effective amount, they provide a combination ofimproved densification to the composition, while providing a fine grainsize in the densified material. Preferably, the sum of WC+Co is at least2.5 w/o, and more preferably, at least 3 w/o if the material is to bedensified by uniaxial hot pressing. If the material is to be densifiedby cold compaction, followed by sintering, preferably the sum of Co+WCshould be at least 3 w/o, and more preferably, at least 3.5 w/o, toassure that adequate densification (i.e., at least 97% of theoreticaldensity) takes place at temperatures of about 2200° C. or less. As thecontent of WC+Co in the present invention increases significantly aboveabout 12 w/o, its wear rate when machining titanium alloys alsoincreases. Therefore, it is preferred that the WC+Co content beminimized to avoid excessive wear rates. Preferably, WC+Co contentshould be less than about 12 w/o, and more preferably, less than 10 w/o.

In accordance with one preferred embodiment of the present invention,about 3.0 to 10 weight percent of WC+Co are added to the TiB₂ powder(or, alternately, ZrB₂, HfB₂, or their solid solutions with each otherand/or TiB₂) and mixed together to form a mixture. Preferably, 0.25 to 1v/o BN may also be added to further control grain growth. The powdermixture is then pressed, preferably at room temperature, to form acompact. The compact is then sintered at a pressure up to 30,000 psi toproduce a substantially fully dense (i.e., at least 97% dense) articleof manufacture, preferably having an average grain size of 8 μm, orless, more preferably, 6 μm or less, and most preferably, 4 μm or less.

These and other aspects of the present invention will become moreapparent upon review of the figures briefly described below inconjunction with the detailed description of the invention whichfollows.

BRIEF DESCRIPTION 0F THE DRAWINGS

FIG. 1 shows an embodiment of a cutting tool in accordance with thepresent invention.

FIG. 2 shows an embodiment of the microstructure of the presentinvention as obtained by a scanning electron microscopy back scatteredimaging technique.

FIG. 3 shows an embodiment of the microstructure of the presentinvention at five times the magnification used in FIG. 2.

FIG. 4 is a graph of nose wear against cutting time for the presentinvention and a prior art uncoated cemented carbide tool during theturning of a Ti-6Al-4V alloy.

FIG. 5 is a graph of maximum flank wear as a function of cutting time inthe turning of a Ti-6Al-4V alloy for the present invention and a priorart uncoated cemented carbide.

FIG. 6 is a graph of maximum flank wear as a function of cutting time inthe machining of Ti-6Al-4V alloy for the present invention and the priorart uncoated cemented carbide at 152 and 213 surface meters/minute (500and 700 surface feet/minute).

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of an article of manufacture in accordance withthe present invention is shown in FIG. 1. While the present inventionmay find use in many applications, the present inventors have found itto be particularly useful as a cutting tool.

FIG. 1 shows an embodiment of an indexable metalcutting insert 10composed of the ceramic material discovered by the present inventors.The present invention is preferably used in the high speed (≧400 surfacefeet/minute) chip forming machining (e.g., turning, milling, grooving,threading, drilling, boring, sawing) of Group IVB metallic materials(i.e., zirconium and its alloys, titanium and its alloys, and hafniumand its alloys). The inventors have found the present invention to beparticularly useful in the high speed machining of titanium alloys.Preferably, the speed should be at least 500 sfm, and preferably, 1,000sfm or less, to obtain the most advantageous use of the presentinvention when machining these materials. Preferred feed ratescontemplated for machining titanium alloys are 0.002 to 0.015inch/revolution, and more preferably, 0.002 to 0.010 inch/revolution.Preferred depths of cut contemplated for machining titanium alloys areabout 0.01 to about 0.2 inch, and more preferably, about 0.01 to about0.15 inch.

The cutting tool 10 has a rake face 30 over which chips formed duringsaid high speed machining of a Group IVB metallic material flow. Joinedto the rake face 30 is at least one flank face 50. At at least onejuncture of the rake face 30 and flank faces 50, a cutting edge 70 isformed, for cutting into the Group IVB metallic material.

While the cutting edge 70 may be in a sharp, honed, chamfered, orchamfered and honed condition, it is preferred that it be in a chamferedcondition, an embodiment of which is illustrated in FIG. 1.

Preferably, the cutting insert 10 has a cutting edge lifetime of atleast 3 minutes, and more preferably, at least 5 minutes during the highspeed machining (e.g., turning) of a titanium alloy. In addition, thetool in accordance with the present invention has a maximum flank wearrate preferably no greater than one-half, and more preferably, nogreater than one-third that of an uncoated cemented carbide tool whenmachining (e.g., turning) a titanium alloy under the same high speedcutting conditions, including flood cooling.

The cutting tool shown in FIG. 1 is, preferably, composed of the TiB₂based ceramic material in accordance with the present invention. FIGS. 2and 3 show typical microstructures of a preferred embodiment (seeExample No. 1, Table I) of the present invention at two differentmagnifications. From FIG. 2, it can be seen that the grain structure issubstantially fine and uniform, with an average grain size estimated tobe about 4 μm. It can be most clearly seen in FIG. 3 that the grainstructure is typically characterized by a dark central phase, or centralportion, which may, preferably, be TiB₂ or Ti_(x1) M_(y1) B₂ where M maypreferably include W and/or Co and y ≧0. This first phase appears to beembedded in a matrix composed of a second, and possibly, a third phase.In many instances, adjacent to and substantially surrounding the centralgrain is a light gray second phase which is believed to be composed of aTi_(x2) M_(y2) Z phase, where x₂ >y₂ and y₂ >0, y₂ /x₂ >y₁ /x₁ and Mpreferably includes W and/or Co. Around many of these phases is a thirdphase which is of a shade of gray intermediate that of the centralportion and the second phase. This third phase is believed to becomposed of a Ti_(x3) M_(y3) Z phase, where M_(y3) is preferably Wand/or Co and where x₃ >Y₃, Y₃ >0 and y₂ /x₂ >y₃ /x₃ >y₁ /x₁ (e.g., thesecond phase has a greater concentration of tungsten in it than thethird phase matrix). The concentration of titanium, however, ispreferably greatest in the central portion of the grain. X-raydiffraction analysis has also shown that the major phase(s) present isof the TiB₂ type crystal structure; however, because of the lack ofsensitivity of x-ray diffraction to minor levels of phases and minorlevels of solid solutioning, it is unclear, based on x-ray diffractionalone, as to what minor phases or solid solutions may be present.

It is also unclear from the x-ray diffraction work alone what phasesform the second and third phases mentioned above. However, since fromthe photomicrographs there appears to be substantial amounts of thesecond and third phases present, their absence from the x-raydiffraction studies done is believed to be explainable if these phasesare also Ti_(xn) M_(yn) B₂ (i.e., Z=B₂) phases containing minor amountsof W and/or Co in solid solution (e.g., Ti_(x2) W_(y2) B₂ and Ti_(x3)W_(y3) B₂). In this case, their absence from the x-ray diffraction tracewould be explained by the almost identical lattice constants they wouldhave with TiB₂. That is, the TiB₂ peaks are substantially identical to,and therefore mask, the peaks of the second and third phases.

While it is believed that the phases forming the halos about the firstphase (see FIG. 3) are diborides, they may also possibly contain minoramounts of, boroncarbide, boronitride, boronoxide, borocarbonitride,boroxycarbide, boroxynitride or a boroxycarbonitride; however, this hasnot been confirmed. What appears to be definite, however, is that theinner halo, or second phase, has a greater concentration of tungstenthan the outer halo, or third phase, and all three phases containtitanium as the major metallic element present.

In addition to the TiB₂ phase observed by x-ray diffraction, otherphases that have been at times observed in minor amounts by x-raydiffraction include CoW₂ B₂, CoWB₅, WB, W₂ B, W₃ CoB, TiB and Ti₃ B₄.The white phase visible in FIG. 3 is believed to be one of the tungstenrich phases mentioned above. The black spots shown in FIG. 3 arebelieved to be porosity.

In the alternative, similar compositions may be made based on ZrB₂ orHfB₂, or their mixtures and solid solutions with each other or TiB₂.These compositions are less preferred than the TiB₂ based compositiondescribed above because of their higher cost. In general, therefore, itcan be stated that the present invention includes a densifiedcomposition, including a first metal diboride phase having a first metalselected from the group of titanium, hafnium and zirconium alone, or incombination with each other, and optionally in combination with W and/orCo, and preferably a second metal diboride phase having a metal whichincludes W and/or Co, in combination with Ti, Hf and/or Zr. Mo, Nb, Tamay be partially or wholly substituted for the W in the material, whileiron and/or nickel may be partially or wholly substituted for the Co inthe material. In addition, W, Mo, Al and/or Cr may be partiallysubstituted for the cobalt in the material.

The densification of the present invention may be achieved either by hotpressing a blend of the appropriate powders or by cold pressing theblended powders to form a compact which is then sintered and hotisostatically pressed.

These processes will be illustrated by the following discussion directedto TiB₂ based compositions, but it should be understood that thetechniques described are also applicable to ZrB₂ and HfB₂ basedcompositions and their mixtures and solid solutions with each otherand/or TiB₂ in accordance with the present invention.

In accordance with the present invention, a blend of powders isprepared, composed of at least 60 w/o; preferably, at least 75 w/o; morepreferably, at least 85 w/o; and most preferably, at least 90 w/o TiB₂.

It is preferred that the level of TiB₂ utilized should be as high aspossible commensurate with the ability of the composition to bedensified by either the hot pressing or the cold pressing-sintering-hotisostatic pressing route in order to achieve the high wear resistancewhile machining titanium alloys. Applicants have found that TiB₂ hasexcellent resistance to reactivity with titanium during titanium alloymachining and has good thermal conductivity compared with otherceramics; however, it is very difficult to densify, while maintaining afine grain size.

Applicants have surprisingly discovered that TiB₂ based ceramics can bereadily densified if WC and Co are added to the TiB₂ powder blend. TheWC and Co may be added: (1) directly as individual WC (or W and C) andCo powders; or (2) as a result of the attrition of the cemented WC-Comilling media during milling of the TiB₂ powder; (3) as a cemented WC-Copowder; or (4) by a combination of (1), (2) and/or (3). At least 2.5 w/ototal of WC+Co should be added to the TiB₂ powder to assuredensification at 2000° C. or less in hot pressing. Where densificationis to be achieved by cold compaction-sintering and hot isostaticpressing, it is preferred that there be at least 3.0 w/o total of WC+Co.

While not optimized, the inventors have found that the ratio of W/Co ona weight percent basis may be about 9:1 to about 20:1. It has been foundthat the addition of the combination of Co and WC in the minimum amountsindicated significantly improves the ease with which densification isachieved, without an adverse effect on the grain size of the resultingmaterial. It is believed that this effect is due to a low melting pointeutectic alloy formed by the WC and Co during the sintering process. Itis, therefore, believed that W/Co ratios as low as 1:20 may also beuseful and may result in a further lowering of the sintering or hotpressing temperatures required to achieve substantial densification. Thetotal WC+Co addition preferably should be less than about 12 w/o, andmore preferably, less than 10 w/o, since increasing WC+Co contentincreases the observed wear rate during the high speed machining oftitanium alloys.

The inventors have also found that the grain size of the densifiedarticle may be further controlled by the addition of an effective amountof a grain growth inhibitor to the powder blend. The inventors,therefore, prefer to add BN powder to the blend at a preferred level ofabout 0.25 to 1.0 v/o of the powder blend.

Limited amounts (not exceeding about 35 v/o total) of other elementsand/or compounds may be added to the powder blend to improve variousproperties of the material for specific applications. Such additionsthat are now contemplated may include: (1) TiC, ZrC, B₄ C, TaC and Mo₂ Cto improve wear resistance; (2) TiN, TiC to assist in densification.Hafnium diboride and/or zirconium diboride may also be substituted forTiB₂ to improve wear resistance, preferably, the total content of HfB₂and ZrB₂ in the composition is also held below 35 v/o. It is alsocontemplated that a portion of the Co addition may be partially replacedby, or supplemented by, small amounts of W, Fe, Mo, Ni, Al and Cr, andtotally replaced by Fe and/or Ni.

Fracture toughness may be further improved through the use of startingpowders having an elongated or whisker morphology. For example, some ofthe TiB₂ starting powder may be replaced by TiB₂ whiskers, or SiC, B₄ C,TiC, ZrC, TaC or Mo₂ C may be added as elongated particles or whiskers.

The foregoing powders are preferably blended for a time appropriate toprovide the desired pick up in WC and Co from the WC-Co cemented carbidemilling media. Preferably, at least about 2.5 w/o of WC+Co is added tothe blend in this manner.

The blended powder is then densified. If it is densified by uniaxial hotpressing, then the hot pressing temperatures and pressures used arepreferably about 1800-2000° C. and about 1 to 5 Ksi, and morepreferably, 1 to 2 Ksi. It is desirable that the hot pressingtemperature be minimized to minimize grain growth. In order to achievemaximum densification during hot pressing, the pressure should bemaintained sufficiently low during temperature elevation to allow gasesgenerated during heat-up to escape. After these gases have escaped, thefull hot pressing pressure may then be applied.

Alternatively, the powder blend may also be densified by cold compactionto form a green compact, followed by sintering, preferably at 1800 to2200° C., preferably followed by hot isostatic pressing, preferably at1700 to 2100° C., and up to 30,000 psi using argon or helium or otherinert gas, but not nitrogen. This manufacturing route is preferable overthe hot pressing route if equivalent levels of densification and a finegrain size can be achieved for a given composition, since the cuttingand grinding of a hot pressed ceramic billets is avoided, therebyreducing the manufacturing cost.

The inventors believe that the grain size in the densified article isvery important to achieving the best metalcutting properties and,therefore, prefer that the average grain size be 8 μm, or less, morepreferably, 6 μm, or less, and most preferably, 4 μm or less. Theinventors believe that a fine grain size is important because TiB₂ has avery high modulus of elasticity, E, and an anisotropic thermal expansioncoefficient, α, which would tend to reduce the thermal shock resistanceof a ceramic containing large TiB₂ grains. The inventors, however,believe that they have minimized any adverse consequences of theseproperties by maintaining the fineness of the grains, as describedabove, which are believed to be substantially randomly oriented.

The resulting articles made in accordance with the present inventionpreferably have a Rockwell A room temperature hardness of about 94.3 to96.5, more preferably, about 94.7 to 96.0, and most preferably, 95.0 to96.0. Their density is, preferably, at least 97%, and more preferably,at least 98% of the theoretical calculated density. The K_(IC) (Evans &Charles) fracture toughness of these articles is difficult to measure,but is estimated to be (using 300 to 500 gm loads), by the Palmqvistindentation method of fracture toughness measurement, about 3.5 to about4.5 MPam^(1/2). Despite this low mechanical fracture toughness, thearticles in accordance with the present invention have been surprisinglyfound to have excellent toughness during the turning of a titanium alloyas described in the examples which follow. These examples are providedto further illustrate the significant benefit provided by the presentinvention in the high speed machining of titanium alloys.

In accordance with the present invention, articles were made of thecompositions shown in Table I.

                                      TABLE I                                     __________________________________________________________________________                           After             Density                                                     Milling           as % of                                                                             Rockwell                                                                           Grain                                       Milling                                                                            Total Hot Press   Calc. A    Size                      Example                                                                            Starting     Time WC + Co                                                                             Temp (°C.)/                                                                   Density                                                                            Theoretical                                                                         Hardness                                                                           Range                     No.  Material*    (minutes)                                                                          (w/o) Press. (Ksi)                                                                         (g/cc)                                                                             Density                                                                             (R.T.)                                                                             (μm)                   __________________________________________________________________________    1.   .25 v/o BN + .63 v/o                                                                       50    4.3  1900/1 4.619                                                                              99    95.2 1-6                            WC + .07 v/o Co                                                          2.   .25 v/o BN   120   5.1  1850/1 4.554                                                                              98    95.5 1-7                       3.   .25 v/o BN   120   5.1  1900/1 4.611                                                                              99    94.9 nm                        4.   .25 v/o BN + 1.28 v/o                                                                      50   ≃7.sup.1                                                              1900/1 4.662                                                                              99    95.2 0.5-7                          WC + .15 v/o Co                                                          5.   .25 v/o BN   45    2.5  1900/1 4.489                                                                              98    94.5  2-17                     6.   .25 v/o BN + 2 v/o                                                                         50   ≃9.2.sup.1                                                            1900/1 4.689                                                                              98    95.4 1-8                            WC + 0.08 v/o Co                                                         7.   .63 v/o WC + 5007 v/o Co                                                                        ≃4.3.sup.1                                                            1900/1 4.589                                                                              99    95.5  1-10                     __________________________________________________________________________     *Remainder TiB.sub.2 and impurities                                           .sup.1 estimated                                                         

The titanium diboride starting powder used was Grade F obtained fromHermann C. Starck Berlin GmbH & Co. KG, P.O.B. 1229, D-7887Laufenburg/Baden, Germany. This powder is composed of crushed and milledirregular shape particles having an hexagonal crystal structure. Thespecification, along with an example of the actual properties for thisgrade of TiB₂ powder, are shown in Table II.

                  TABLE II                                                        ______________________________________                                        Specification         Measured Property                                       ______________________________________                                        BET Specific Surface Area >4 m.sup.2 /g                                                              4.  m.sup.2 /g                                         Scott TAP Density/Apparent Density--                                                                 9.2 g/in.sup.3                                         FSSS Particle Size max. 0.9 μm                                                                    0.9 μm                                              Max. Particle Size 98% <6 μm                                                                     --                                                      Ti           ≧66.5 wt. %                                                                          remainder                                          B            ≧28.5 wt. %                                                                           29.8                                              Nonmetallic Impurities                                                        C            ≦0.25 wt. %                                                                           0.21                                              O            ≦2.0 wt. %                                                                            1.92                                              N            ≦0.25 wt. %                                                                           0.11                                              Metallic Impurities                                                           Fe           ≦0.25 wt. %                                                                           0.18                                              Other - Total                                                                              ≦0.2 wt. %                                                                           <0.2                                               ______________________________________                                        The boron nitride starting powder was                                         obtained from Union Carbide as grade HCP.                                     The WC powder had the following properties:                                   Total Carbon  6.11 w/o   Cr        .01 w/o                                    Free Carbon    .01 w/o   Ta        .12 w/o                                    O.sub.2        .17 w/o   Ca        .21 w/o                                    Ni             .01 w/o   Fe        .02 w/o                                                             BET      1.36 m.sup.2 /g                             ______________________________________                                         The cobalt powder was an extra fine grade cobalt.                        

These powders were milled together in the ratios shown in Table I toform 100 gm lots. Wet milling was performed in a polyurethane lined ballmill with isopropanol and about 3900 gm of WC-Co cemented carbidecycloids for the times shown in Table I. These cemented carbide cycloidshave a nominal composition containing about 5.7 w/o Co, 1.9 w/o Ta and anominal Rockwell A hardness and nominal magnetic saturation value ofabout 92.7 and about 92 percent, respectively.

From our experience in milling these powders under the conditionsdescribed, it is estimated that, for a milling time of 45 to 50 minutes,about 2.4 to about 2.7 w/o of WC+Co, and for a milling time of 120minutes, about 4.1 to about 5.8 w/o WC+Co, is added to the blend due tothe attrition of the WC-Co cemented carbide cycloids during milling.

After milling, powder blends were dried, screened and then uniaxiallyhot pressed according to the conditions shown in Table I in an argonatmosphere. During heating, pressure was not applied. The pressingpressure was first applied at the hot pressing temperature and held fortypically one hour. The resulting articles produced were essentiallyfully dense and have the densities, hardnesses and grain sizes shown inTable I. Billets made in accordance with Example 1 were cut and groundto produce SNGN-453T (0.002-0.004 inch×20° chamfer) style indexablemetalcutting inserts (see FIG. 1).

These inserts were tested in the metalcutting tests described below inTable III against prior art sharp edged K313 grade cemented carbideSNGN-433 style cutting inserts. These tests were run under flood coolantat 600, 800 and 1,000 surface feet/minute, 0.005 ipr and 0.050 inchdepth of cut as described in Table III. The cutting tools composed ofmaterial in accordance with the present invention had more than twicethe life of the prior art cemented carbide tools. It was observed thatchemical reaction between the titanium alloy workpiece and the cuttingtool according to the present invention was the dominant wear mechanismon the rake and flank faces. The present invention, however, had asignificantly lower wear rate than the prior art tool, as illustrated bythe graphs of nose wear and maximum flank wear shown in FIGS. 4 and 5,respectively, which are based on the results of the 600 sfm test shownin Table III.

A water soluble coolant, which may be used in these applications, isCimtech 500. Cimtech 500 is a synthetic fluid concentrate for machiningand stamping ferrous metals. It is supplied by Cincinnati MilicronMarketing Co., of Cincinnati, Ohio. It is typically diluted in water ata water to coolant ratio of 30:1 to 20:1 for machining applications.

                  TABLE III                                                       ______________________________________                                        METALCUTTING TEST RESULTS                                                             Tool Life in Minutes                                                          600 sfm   800 sfm   1,000 sfm                                         ______________________________________                                        Invention >5.0        >1.5      1.5 BK                                        Prior Art 2.5 FW      <0.5 BK   Not Run                                       Conditions                                                                    Operation:     Turning                                                        Workpiece Material:                                                                          Ti-6Al-4V titanium alloy (Annealed                                            at 1300° F. for 2 hours and air                                        cooled                                                         Cutting Speed: as noted above                                                 Feed:          .005 inch/revoluation                                          Depth of Cut:  .050 inch                                                      Lead Angle:    45 degrees                                                     Rake Angle:    -5 degrees back rake and -5                                                   degrees side rake                                              Cutting Fluid: Flood water soluble coolant diluted                                           20:1 with water                                                End of Life Criteria                                                          Breakage (BK)                                                                 Flank Wear (FW)        ≧.030 inch                                      Maximum Flank Wear (MW)                                                                              ≧.040 inch                                      Nose Wear (NW)         ≧.040 inch                                      Depth of Cut Notching (DN)                                                                           ≧.080 inch                                      ______________________________________                                    

It was further surprisingly found that the low fracture toughness,mentioned above, of the invention did not adversely affect the abilityof the material to turn the above titanium alloy. It was furthersurprisingly found that the use of flood coolant did not cause theinvention to break from excessive thermal shock. These resultsdemonstrate that the present invention has at least twice the cuttingedge lifetime of the prior art cemented carbide tools at machiningspeeds far beyond those recommended (i.e., <250 sfm) for uncoatedcarbide.

While uncoated cemented carbides can achieve similar lifetimes to thosefound for the present invention at lower speeds (<250 sfm), these lowerspeeds greatly reduce the metal removal rate, which is important indetermining machining costs and machine availability.

In another example, the composition used in Example 1 was blended inaccordance with Example 1. After milling, the powders were dried,pelletized with a lubricant/fugitive binder (e.g., rosin/polyethyleneglycol) and then uniaxially cold pressed to form green cutting inserts.The green inserts were heated in a vacuum up to about 460° C. tovolatilize the lubricant and fugitive binder. Heating was then continuedin one atmosphere of argon to a sintering temperature of about 2000° C.,which was held for 60 minutes and then cooled to room temperature. Thesintered inserts were then hot isostatically pressed at 1850° C. for 60minutes under 15 ksi argon. Sintering and hot isostatic pressing wereperformed by placing the inserts on a bed of boron nitride settingpowder. The inserts were then ground to final size. In this manner,RNGN-45T (0.002-0.004 inch×20° chamfer) style cutting inserts werefabricated. These cutting inserts were tested against prior art K313grade cemented carbide cutting inserts in the RNGN-45 style with a sharpcutting edge in the turning of Ti-6Al-4V titanium alloy. The testconditions, as well as the results of these tests, are shown in Table IVand FIG. 6, and are summarized below:

A single test trial was run at 152 m/minute (500 sfm), comparing thepill pressed-sinter-Hipped inserts to the prior art K313 grade ofcemented carbide. FIG. 6 is a plot of maximum flank wear. The importantobservation is that the wear rate for the present invention isrelatively uniform through the end of life, at 10 minutes (based on0.040 inch maximum flank wear). The 152 m/minute cutting speed is toohigh for the prior art, which had less than three minutes of tool lifedue to maximum flank wear exceeding 0.040 inch.

                  TABLE IV                                                        ______________________________________                                                   MAXIMUM FLANK WEAR (inch)                                                 Time  RUN 1         RUN 2                                              CON-     (min-   INVEN-   PRIOR  INVEN- PRIOR                                 DITION   utes)   TION     ART    TION   ART                                   ______________________________________                                        500 sfm/ 1       .0151    .0081                                               .0072 IPR/                                                                             2       .0169    .0203                                               .050" DOC/                                                                             3       .0182    .0428                                               Flood    4       .0214                                                        Coolant  5       .0223                                                                 6       .0255                                                                 7       .0286                                                                 8       .0301                                                                 9       .0345                                                                 10      .0401                                                        700 sfm/ 1       .0163    .0263  .0170  .0354                                 .0072 IPR/                                                                             2       .0228    .0984  .0230  .0510                                 .050" DOC/                                                                             3       .0285           .0296                                        Flood    4       .0367           .0359                                        Coolant  5       .0445           .0434                                        1000 sfm/                                                                              1       .0202    .1436                                               .005 IPR/                                                                              2       broken                                                       .050" DOC/                                                                    Flood                                                                         Coolant                                                                       ______________________________________                                    

Two test trials were run at 213 m/minute (700 sfm) (see Table IV andFIG. 6). It was found that the pill pressed-sintered and Hipped inserts(Δ) had equal or better wear compared to the hot pressed inserts (□)under these conditions. (The hot pressed insert (RNGN-43T) failedprematurely by cracking because it was too thin for this application.)At 700 sfm, the present invention maintains a uniform wear rate that issignificantly superior to that produced in the prior art cutting tool.The 700 sfm cutting speed is also clearly out of the useful range of theuncoated carbide tested, which experienced extreme localized wear inless than two minutes.

In an attempt to determine the upper limit of the cutting speed for thepresent invention, a test was run at 1000 sfm (see Table IV). Theinvention failed at two minutes due to breakage. The prior art cementedcarbide tool experienced extreme localized wear and resulting rake andflank face chipping in less than one minute.

Based on the foregoing examples, it is clear that the pill pressed,sintered and Hipped cutting tools according to the present inventionhave the same capabilities for machining titanium based materials as thehot pressed cutting tools according to the present invention. Thepresent invention is capable of withstanding cutting speed which aresignificantly beyond the useful operating range of uncoated cementedcarbide. It was further found that the present invention can withstandlarger wear scars without experiencing the acceleration in wear ratethat is typical cemented carbide cutting tools.

It is further believed that the metalcutting performance of the presentinvention may be further improved, allowing longer cutting edge lifetimeand/or higher machining speed capabilities, through the application of arefractory coating to the rake face, flank face and cutting edge. Thecoating may be applied by known PVD or CVD techniques now used to coatcutting tools. A refractory coating having one or more layers ispreferably composed of one or more of the following refractorymaterials: alumina, and the borides, carbides, nitrides andcarbonitrides of zirconium, hafnium and titanium, their solid solutionswith each other and their alloys. It is further proposed that use ofsuch a refractory coating may allow the use of higher levels oftoughening agents or WC+Co to further improve the sinterability of thepresent invention, while minimizing the adverse impact of such increaseson the wear rate when machining titanium alloys.

It is also contemplated that cutting inserts in accordance with thepresent invention may be fabricated with either a ground in, or molded,chipbreaker structure. Examples of chipbreaker structures which may beused herein are described in U.S. Pat. No 5,141,367. Titanium alloychips are notoriously hard to break. This may be partially due to theslow speeds used when uncoated cemented carbides are utilized to turntitanium alloys. It is our belief that the higher machining speeds nowpossible with the present invention, in combination with a chipbreakerstructure, may lead to improve chip control during the turning oftitanium alloys.

All patents and other publications referred to herein are herebyincorporated by reference in their entireties.

Other embodiments of the invention will be apparent to those skilled inthe part from a consideration of this specification or practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with the true scope and spiritof the invention being indicated by the following claims.

What is claimed is:
 1. A metalcutting tool for the chip formingmachining of a group IVB metallic material comprising:a rake face overwhich said chips will flow during said machining of said group IVBmetallic material; a flank face and a cutting edge, for cutting intosaid group IVB metallic material at high speeds, formed at a juncture ofsaid rake and said flank face; wherein said metal cutting tool has aceramic composition consisting essentially of a Group IVB boride phase.2. The metalcutting tool according to claim 1 wherein the compositionhas an average grain size of 8 μm or less.
 3. The metalcutting toolaccording to claim 1 wherein the Group IVB boride phase is a titaniumboride phase.
 4. The metalcutting tool according to claim 2 wherein theGroup IVB boride phase is a titanium boride phase.
 5. A cutting toolcomprising:a rake face; a flank face; and a cutting edge formed at ajuncture of said rake face and said flank face; wherein said cuttingtool is characterized by:a microstructure consisting essentially ofphases having a TiB₂ crystal structure; and wherein said phases includephases containing tungsten at differing concentration levels; a densityof at least 97% of theoretical density; a hardness of 94.3 to 96.5Rockwell A at room temperature; and wherein said microstructure has anaverage grain size of 8 μm or less.
 6. The metalcutting tool accordingto claim 1 wherein said ceramic composition is made from at least 75weight percent titanium diboride.
 7. The metalcutting tool according toclaim 2 wherein said ceramic composition is made from at least 75 weightpercent titanium diboride.
 8. The metalcutting tool according to claim 1wherein said ceramic composition is made from at least 85 weight percenttitanium diboride.
 9. The metalcutting tool according to claim 2 whereinsaid ceramic composition is made from at least 85 weight percenttitanium diboride.
 10. The metalcutting tool according to claim 1wherein said ceramic composition is made from at least 90 weight percenttitanium diboride.
 11. The metalcutting tool according to claim 2wherein said ceramic composition is made from at least 90 weight percenttitanium diboride.
 12. The metalcutting tool according to claim 1wherein said composition has an average grain size of 6 μm or less. 13.The metalcutting tool according to claim 1 wherein said composition hasan average grain size of 4 μm or less.
 14. The metalcutting toolaccording to claim 6 wherein said composition has an average grain sizeof 6 μm or less.
 15. The metalcutting tool according to claim 8 whereinsaid composition has an average grain size of 6 μm or less.
 16. Themetalcutting tool according to claim 10 wherein said composition has anaverage grain size of 6 μm or less.
 17. The metalcutting tool accordingto claim 6 wherein said composition has a density of at least 98 percentof the theoretical calculated density.
 18. The metalcutting toolaccording to claim 8 wherein said composition has a density of at least98 percent of the theoretical calculated density.
 19. The metalcuttingtool according to claim 10 wherein said composition has a density of atleast 98 percent of the theoretical calculated density.
 20. Themetalcutting tool according to claim 12 wherein said composition has adensity of at least 98 percent of the theoretical calculated density.21. The metalcutting tool according to claim 14 wherein said compositionhas a density of at least 98 percent of the theoretical calculateddensity.
 22. The metalcutting tool according to claim 15 wherein saidcomposition has a density of at least 98 percent of the theoreticalcalculated density.
 23. The metalcutting tool according to claim 16wherein said composition has a density of at least 98 percent of thetheoretical calculated density.
 24. The cutting tool according to claim5 wherein said density is at least 98 percent.
 25. The cutting toolaccording to claim 24 wherein said microstructure is made from at least75 weight percent titanium diboride.
 26. The cutting tool according toclaim 24 wherein said microstructure is made from at least 85 weightpercent titanium diboride.
 27. The cutting tool according to claim 24wherein said microstructure is made from at least 90 weight percenttitanium diboride.